1. Field of the Invention
The present disclosure is generally related to machine tooling apparatus and methods and more particularly, to multi-step drilling apparatus and methods using air flow sensing for controlling the sequence of steps in a multi-step drilling process.
2. Related Art
In the manufacturing of aircraft, the aerospace industry requires the drilling of thousands of fastener holes, used, for example, in attaching an aircraft skin to an airframe or a wing to an aircraft body. Typically, at present this drilling may be accomplished through the use of automated, semi-automated, or manual drill motors depending on the specific requirements of the parts being drilled.
A multi-step machining operation is particularly useful when machining composite/metal stack materials, which are generally workpieces consisting of multiple layers of carbon fiber-reinforced plastic (CFRP) and aluminum or hard metals (titanium, steel, etc.). Multi-layered components may also be preassembled in positioning jigs or fixtures with fastener holes being drilled through the assembly in a single pass, thus improving the accuracy of the positioning of the holes of the layers in the assembly.
In general, multiple steps are required in critical areas where the hole quality specifications (total cylindricity, surface roughness and carbon fiber delimitation) cannot be met with a single step process. A single step process is always desired but often it is physically impossible to generate a hole of the desired diameter and quality with a single pass. Mixed material stacks are generally the most difficult to drill but often holes in single material stacks must be drilled with multiple steps to achieve the desired hole quality.
In practice, each step in a sequence will result in a larger hole than the previous step until the desired final hole size is reached. This can mean that some steps will only drill some portion of the total material stack due to existing holes in some layers but often all steps drill the entire stack. Often in a two-step process, the first step results in a hole that is only slightly smaller than the desired final hole size to remove the majority of the material. In general, the resulting hole quality for the first step will be poor because it will be optimized for speed as opposed to quality. The second and final step will produce the desired final hole size and be optimized for hole quality. Because the second step removes a relatively small amount of material, it is easier to optimize to achieve superior hole quality.
Thus, machining a composite/metal stack will usually require a multi-step process, where each step might drill one, several, or all layers of the total material stack. The multi-step process may be performed by two or more drilling units where each of the drilling units performs at least one step of the multi-step process.
An advantage of utilizing multiple drilling units in a multi-step process is that each drilling unit may be configured to optimize its machining operation, e.g., by changing the cutting tool. However, each drilling unit must perform its particular machining operation in the proper sequence relative to each hole being drilled. Accordingly, there is a need for apparatus and methods that will ensure that when using multiple drilling units, each drilling unit will perform its machining operation on each hole drilled in the proper sequence relative to the other drilling units, such that each drilling unit will not be allowed to perform its machining operation out of sequence.